Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal; from feed stock intermediates, such as syngas; or from starchy materials or cellulosic materials, such as corn or sugar cane. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulosic materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol. When feed stock prices rise, the need for alternative sources of ethanol production becomes more evident. Starchy materials, as well as cellulosic materials, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol for fuels or consumption. In addition, fermentation of starchy or cellulosic materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
Ethanol production via the reduction of alkanoic acids and/or other carbonyl group-containing compounds has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature. During the reduction of alkanoic acid, e.g., acetic acid, other compounds are often formed with ethanol or are formed in side reactions. For example, during hydrogenation, esters are produced that together with ethanol and/or water form azeotropes, which are difficult to separate. These impurities may limit the production of ethanol and may require expensive and complex purification trains to separate the impurities from the ethanol. Also, the hydrogenation of acetic acid typically yields ethanol and water along with small amounts of side reaction-generated impurities and/or by-products. At maximum theoretical conversion and selectivity, the crude ethanol product would comprise approximately 72 wt. % ethanol and 28 wt. % water. In order to form purified ethanol, much of the water that is co-produced must be removed from the crude ethanol composition. In addition, when conversion is incomplete, unreacted acid may remain in the crude ethanol product. It is typically desirable to remove this residual acetic acid from the crude ethanol product to yield purified ethanol.
It is also well known to reduce, e.g., hydrogenate, aldehydes to their corresponding alcohol. Thus, ethanol may be formed via the hydrogenation of acetaldehyde. Exemplary aldehyde hydrogenation processes are described in U.S. Pat. Nos. 5,093,534; 5,004,845; 4,876,402; 4,762,817; 4,626,604; 4,451,677; 4,426,541; 4,052,467; 3,953,524; and 2,549,416, the entireties of which are incorporated herein by reference.
As an example, crotonaldehyde may be hydrogenated to form crotyl alcohol. The following references relate to this reaction: (1) Djerboua, et al. “On the performance of a highly loadedCO/SiO2 catalyst in the gas phase hydrogenation of crotonaldehyde thermal treatments—catalyst structure-selectivity relationship,” Applied Catalysis A: General (2005), 282, pg 123-133; (2) Liberkova, and Tourounde, “Performance of Pt/SnO2 catalyst in the gas phase hydrogenation of crotonaldehyde,” J. Mol. Catal. A: Chemical (2002), 180, pg. 221-230; (3) Rodrigues and Bueno, “Co/SiO2 catalysts for selective hydrogenation of crotonaldehyde: III. Promoting effect of zinc,” Applied Catalysis A: General (2004), 257, pg. 210-211; (4) Ammari, et al., “An emergent catalytic material: Pt/ZnO catalyst for selective hydrogenation of crotonaldehyde,” J. Catal. (2004), 221, pg. 32-42; (5) Ammari, et al., “Selective hydrogenation of crotonaldehyde on Pt/ZnCl2/SiO2 catalysts,” J. Catal. (2005), 235, pg. 1-9; (6) Consonni, et al. “High Performances of Pt/ZnO Catalysts in Selective Hydrogenation of Crotonaldehyde,” J. Catal. (1999), 188, pg. 165-175; and (7) Nitta, et al., “Selective hydrogenation of αβ-unsaturated aldehydes on cobalt-silica catalysts obtained from cobalt chrysotile,” Applied Catal. (1989), 56, pg. 9-22.
Even in view of these teachings, the need remains for improved processes for producing ethanol via acetaldehyde hydrogenation, which have high ethanol production efficiencies.